This invention relates to a DC arc welding apparatus with a switching element.
A conventional DC arc welding apparatus of this type is shown in FIG. 1, wherein reference numeral 1 designates a DC power source including a welding transformer, 2 is a switching element for controlling the output current of the power source including a single power transistor or a plurality thereof connected in parallel, 3 and 4 are the output terminals of the welding apparatus, and 5 is a welding electrode connected to the output terminals 3. A material 7 to be welded (hereinafter referred to as "a base metal") is connected to the output terminal 4, and arcs 6 are established between the electrode 5 and the base metal. A current detecting element 8 such as a shunt resistor is coupled to an output current feedback circuit 9 for amplifying the output signal of the detecting element. A variable resistor 10 across which a constant voltage is applied provides an output current setting signal to a current control circuit, to which the output of feedback circuit 9 is also applied. The current control circuit 11 compares its two inputs, and their amplified difference is supplied to a drive circuit 12 whose output controls the on-off operation of the switching element 2. Reference numerals 13 and 14 designate a flywheel diode and a DC reactor, respectively.
The feedback circuit 9, as shown in FIG. 2, comprises resistors R.sub.1 and R.sub.2 and an amplifier IC.sub.1. The current detected by element 8 is thus amplified R.sub.2 /R.sub.1 times, to provide an output signal e.sub.if.
The current control circuit 11, as shown in FIG. 3, comprises resistors R.sub.3 through R.sub.7, an amplifier IC.sub.2, a diode D.sub.1, a light emitting diode D.sub.2, and a photo-transistor TR.sub.1, the l.e.d. and the photo-transistor forming a photo-coupler. If the resistance of R.sub.3 is equal to that of R.sub.4, the value applied to the non-inverting input terminal of the amplifier IC.sub.2 will be (e.sub.if +e.sub.is)/2. If the output of the amplifier is represented by e.sub.2, then the value e.sub.1 applied to the inverting input terminal of the amplifier is: ##EQU1##
Accordingly, when (e.sub.if +e.sub.is)/2&gt;e.sub.1, then e.sub.2 &lt;0; and when (e.sub.if +e.sub.is)/2&lt;e.sub.1, then e.sub.2 &gt;0. The comparison signal e.sub.1 depends on the value of e.sub.2, however, and thus the signal e.sub.if from feedback circuit 9 when e.sub.2 changes from positive to negative is different in value from the signal e.sub.if when e.sub.2 changes from negative to positive, which results in a hysteresis characteristic. Accordingly, when the output e.sub.2 of the amplifier IC.sub.2 is positive (e.sub.2 &gt;0), a forward current flows in the light emitting diode D.sub.2 to drive the photo-transistor TR.sub.1.
The drive circuit 12, as shown in FIG. 4, comprises a power source E, resistors R.sub.8 through R.sub.13, and transistors TR.sub.2 through TR.sub.5, these circuit elements forming a multi-stage transistor connection circuit. In response to the operation of the photo-transistor TR.sub.1 in the current control circuit, the drive circuit 12 outputs a signal e.sub.6 to operate (on and off) the switching element 2.
In the operation of the conventional DC arc welding apparatus shown in FIG. 1, when the switching element 2 is rendered conductive (on) an output current flows, in order, from the DC power source 1 through the element 2, the DC reactor 14, the output terminal 3, the electrode 5, the arc 6, the base metal 7, the output terminal 4, and the current detecting element 8 back to the DC power source.
When the switching element 2 is rendered non-conductive (off), current flows in a loop consisting of the DC reactor 14, the output terminal 3, the electrode 5, the arc 6, the base metal 7, the output terminal 4, the current detecting element 8 and the flywheel diode 13 because of the energy storage effect of the reactor. Furthermore, owing to such effect of the reactor, the current rise and fall characteristic in rendering the switching element 2 conductive and non-conductive has a predetermined time constant. The output signal of the current detecting element 8 is amplified in the feedback circuit 9. At the same time, in the current control circuit 11, the signal e.sub.if from the feedback circuit and the signal e.sub.is set by the variable resistor 10 are subjected to comparison and amplification, as a result of which two different "high" and "low" signals are selectively applied to the drive circuit 12.
In response to the "high" signal the drive circuit operates to render the switching element 2 conductive, and vice versa. Thus, if a current I.sub.ON which causes the switching element 2 to be rendered conductive and a current I.sub.OFF which causes the switching element to be rendered non-conductive are controlled by the circuit 11, having a hysteresis characteristic, such that I.sub.ON &lt;I.sub.OFF is satisfied as shown in FIG. 5, then a predetermined current can flow in the arc region 6. The output current flowing in the arc region 6 can be varied by changing the setting of the variable resistor 10.
The conventional DC arc welding apparatus described above has a constant current characteristic whereby the output current remains constant even if the load is changed, and is therefore applicable to the field of TIG welding, for example, employing a non-consumable electrode. In most cases where consumable electrodes are used, a welding apparatus having a constant voltage characteristic is employed in which the welding is stable with the arcs being self-controlled and maintained constant in length. With a welding apparatus having a constant current characteristic it is difficult to self-control the arcs, and even if the arc length is changed manually by shaking or the like, it is difficult to restore the arc length change in a short time. Accordingly, when the electrode is consumable it is necessary that the arc length change is detected from the arc voltage in order to control the feed rate of the consumable electrode. Most electrode feed motors have a slow response characteristic, however, and it is therefore necessary to employ an expensive pulse motor or the like which has an excellent response characteristic.
Furthermore, the employment of a constant current welding apparatus in a consumable electrode type of welding operation is disadvantageous in that the circuitry involved is intricate and therefore high in manufacturing cost.